This invention relates to powder feeding apparatus, and, more particularly, to an apparatus for feeding particulate powder material having a filtered vent.
Particulate powder coating materials are commonly used to coat or paint objects in industrial finishing applications. In such applications, a particulate powder material such as epoxy, polyester or porcelain frit is conveyed to an applicator gun while entrained in an airstream and is sprayed from the nozzle of the gun onto a target surface or a substrate. Conventionally, the target substrate having powder loosely adhered thereto is then heated to melt the powder so that when the melted powder cools, it is permanently adhered to the substrate.
In most applications, powder deposition is performed in a booth which mounts the applicator guns. Articles are moved through the booth on an overhead conveyor and coated with particulate powder material supplied to the applicator guns by a source of air-entrained powder. Oversprayed powder is contained within the booth by an exhaust system and collected in a powder recovery system where it is either held or recirculated back to the applicator guns, generally via a powder feed hopper.
Powder feed hoppers which supply the air-entrained, particulate powder material to the applicator or spray guns contained in the powder spray booth generally comprise a housing having an inlet to receive recirculated, oversprayed powder from the booth, and an outlet connected to a powder pump. In many prior art powder feeders, a fluidized bed is mounted at the base of the housing which is supplied with fluidizing air from a pressurized air source, Powder introduced into the housing through the inlet is received atop the fluidized bed where an upward flow of pressurized air through the fluidizing plate fluidizes the powder in a well known, commercially practiced manner. The powder pump draws the powder from the fluidized bed and entrains the powder within a stream of air. The air-entrained powder is then transmitted to the applicator guns of the powder spray booth for coating the target articles.
The oversprayed powder from the spray booth is recirculated back to the powder feed hopper by a powder pump which entrains the recirculated powder in a stream of air. This air injected with the oversprayed powder into the interior of the housing must be vented to prevent a pressure buildup within the housing. If the pressure in the interior of the housing was allowed to build, the fluidized bed would be rendered inoperative as soon as the internal pressure in the chamber exceeded the pressure of the low pressure airstream moving upwardly through the fluidizing plate.
In the prior art, a hose or pipe has been connected to the housing of the feed hopper to vent its interior. In many instances, the vent pipe has been left open to atmosphere. This has presented environmental problems, however, because the recirculated powder tends to billow or puff up to some extent when injected into the housing so that at least a portion of the powder is suspended within the interior of the housing. At least some of this free floating powder escapes through the vent directly into the atmosphere which can create both health and safety hazards.
In an effort to reduce the hazards caused by direct venting of the housing to atmosphere, a filter has been placed over the vent pipe connected to the housing of the feed hopper. This filter is usually a bag or sock formed of nylon or other woven material having a sufficiently closed weave to capture the powder particles escaping through the vent.
Although the practice of attaching a filitring material to the vent of the housing is preferable to leaving the vent open to atmosphere, prior art filters present a maintenance problem. The bag or sock filter must be periodically removed from the vent and either cleaned or replaced with a new filter. During such maintenance periods, the powder feeder cannot be operated without injecting particulate powder material into the atmosphere.